1. Field of the Invention
The present invention relates to a laminated piezoelectric actuator and, more specifically, to a laminated piezoelectric actuator used, for example, as a precision positioning device in an optical equipment, as a drive element for preventing vibration and as a drive element for fuel injection in an automotive engine.
2. Description of the Prior Art
There has heretofore been known a laminated piezoelectric actuator comprising a laminate in which plural pieces of piezoelectric layers and plural pieces of internal electrode layers are alternatingly laminated one upon the other. In the piezoelectric actuator of the above-mentioned type, a voltage is applied to the internal electrode layers among which the piezoelectric layers are sandwiched to obtain a large displacement by utilizing the inverse piezoelectric effect that occurs in the piezoelectric layers.
In the laminated piezoelectric actuator, the internal electrode layer laminated on one surface of the piezoelectric layer is used as a first electrode (e.g., positive electrode) and the internal electrode layer laminated on the other surface thereof is used as a second electrode (e.g., negative electrode); i.e., a partial electrode structure is employed in which the internal electrode layers have areas smaller than the areas of the piezoelectric layers.
FIG. 12 is a side sectional view illustrating a conventional laminated piezoelectric actuator having the partial electrode structure, in which plural piezoelectric layers 1 and internal electrode layers 2 are alternatingly laminated to form an actuator body 3, and a pair of external electrodes 4 and 4 are formed on the side surfaces thereof.
As will be obvious from FIG. 12, plural internal electrode layers 2 include first internal electrode layers 2a and second internal electrode layers 2b that are alternatingly laminated one upon the other, and the ends of the first internal electrode layers 2a and the ends of the second internal electrode layers 2b are electrically connected to the external electrode terminals 4a and 4b that are formed on different side surfaces of the actuator body 3. That is, the external electrode 4a is electrically connected to the ends of the first internal electrode layers 2a but is not connected to the ends of the second internal electrode layers 2b. Similarly, the external electrode 4b is electrically connected to the ends of the second internal electrode layers 2b but is not connected to the ends of the first internal electrode layers 2a. Further, the piezoelectric layers sandwiched among the first internal electrode layers 2a and the second internal electrode layers 2b are polarized in the directions of arrows as shown.
Japanese Unexamined Patent Publication (Kokai) No. 147880/1989 discloses a laminated piezoelectric actuator having a structure as shown in FIG. 12, in which insulating blocks of a suitable shape are provided between the external electrode 4a and the second internal electrode layers 2b, and between the external electrode 4b and the first internal electrode layers 2a. The insulating blocks prevent electric conduction between the external electrode 4a and the second internal electrode layers 2b, and between the external electrode 4b and the first internal electrode layers 2a. 
In the laminated piezoelectric actuator of the partial electrode structure shown in FIG. 12, however, distortion occurs due to the inverse piezoelectric effect in the portion where the first internal electrode layer 2a and the second internal electrode layer 2b are overlapped one upon the other (portions where the piezoelectric layers 1 are held by the internal electrode layers 2), but no inverse piezoelectric effect occurs near the side surfaces of the actuator body 3 where the piezoelectric layers 1 are not held by the internal electrode layers 2, and the actuator as a whole produces a small amount of displacement.
In each piezoelectric layer 1, further, distortion due to the inverse piezoelectric effect becomes nonuniform, and stress concentrates near the end of the internal electrode layer 2. Due to the concentration of stress, cracks spread from the end of the internal electrode layer 2 into the piezoelectric layer 1, causing the piezoelectric layer 1 to be broken (for example, see Destruction Mechanisms in Ceramic Multilayer Actuators: Japan Journal Appl. Physics, Vol. 33 (1994), pp. 3091-3094).
In the piezoelectric actuator disclosed in Japanese Unexamined Patent Publication (Kokai) No. 147880/1989, further, the electric field concentrates conspicuously at the end of the internal electrode layer located near the insulating block, stress concentrates inside the piezoelectric layer or in the interface between the piezoelectric layer and the internal electrode layer, causing a mechanical destruction in the actuator body or a destruction in the insulation of the insulating block and, hence, causing the life to be shortened.
It is therefore an object of the present invention to provide a laminated piezoelectric actuator which very little permits the occurrence of insulation breakdown and mechanical break down, and is highly reliable featuring a long life.
According to the present invention, there is provided a laminated piezoelectric actuator comprising (a) an actuator body constituted by plural piezoelectric layers and plural internal electrode layers alternatingly laminated in the direction of height, the internal electrode layers of one side constituting first electrode layers and the internal electrode layers of the other side constituting second electrode layers so as to be neighbored one another with the piezoelectric layers sandwiched among them, (b) external electrodes which are formed on the side surfaces of the actuator body and are connecting the ends of the internal electrode layers, and (c) non-active ceramic layers arranged at an upper end and a lower end of the actuator body; wherein
the external electrodes include a first external electrode connecting the ends of the first electrode layers, and a second external electrode connecting the ends of the second electrode layers and is formed on a side surface of the actuator body different from the side surface on where the first external electrode is formed;
insulating blocks are arranged between the first external electrode and the ends of the second electrode layers, and between the second external electrode and the ends of the first electrode layers;
flat surfaces are formed on the side surfaces of the insulating blocks that are in contact with the ends of the first electrode layers and with the ends of the second electrode layers, the flat surfaces extending in parallel with the side surfaces of the actuator body; and
when the thickness of the internal electrode layers is denoted by t1, the thickness of the piezoelectric layers by t2, and the length of the flat surfaces by L, a relation represented by the following formula,
0.2xe2x89xa6(Lxe2x88x92t1)/t2 less than 1
is satisfied.
That is, the present invention was accomplished by giving attention to the fact that the concentration of the electric field is seriously affected by the shape of the insulating blocks and, particularly, by the shape of the interface between the insulating blocks and the internal electrodes. By forming the insulating blocks in a shape to satisfy the conditions of the above-mentioned formula (1), it is allowed to lower the degree of concentration of the electric field in the piezoelectric layers near the ends of the internal electrode layers located close to the side surfaces of the insulating blocks and, hence, to effectively prevent the breakdown in the insulation of the insulating blocks caused by the concentration of the electric field. Further, the concentration of stress is effectively prevented in the piezoelectric layers or in the internal electrode layers, that is caused by the concentration of the electric field making it possible to effectively prevent the mechanical breakdown of the actuator body. Thus, the present invention enhances the reliability of the laminated piezoelectric actuator and extends the life.
According to the present invention, it is desired to form external electrodes by using an electrically conducting composition which comprises a resin matrix of a heat-resistant resin having a 5%-weight-reduction temperature of not lower than 250xc2x0 C. and at least one kind of electrically conducting agent selected from the group consisting of electrically conducting ceramics, a metal oxide, and a metal of the group of 6 to 11 of periodic table or an alloy thereof. Upon forming the external electrodes by using such an electrically conducting composition, it is allowed to effectively prevent the breakage of connection between the internal electrode layers and the external electrodes caused by the expansion and contraction of the piezoelectric layers during the operation and by a difference in the thermal expansion between the piezoelectric layers and the internal electrode layers and, hence, to obtain a laminated piezoelectric actuator having excellent durability.
Non-active ceramic layers are provided at an upper end and a lower end of the actuator body to maintain the strength and to transmit the displacement of the actuator to the external side. When the actuator is energized, the actuator body expands and contracts but the non-active ceramic layers neither expand nor contract. Accordingly, shearing stress builds up in the boundary portions between the ceramic layers and the actuator body to deteriorate the durability of the actuator. According to the present invention, the actuator body is divided into three regions along the direction of lamination, i.e., into a central portion and stress-relaxing portions over and under the central portions, the non-active ceramic layers are provided neighboring the stress-relaxing portions, and the thickness of the piezoelectric layer included in the stress-relaxing portion is selected to be larger than the thickness of the piezoelectric layer in the central portion in order to suppress the shearing stress occurring in the boundary portion between the non-active ceramic layers and the actuator body and to improve durability of the actuator.